1. Field of the Invention
The present invention relates to FM modulators, and more specifically to an FM modulator for generating a wide-band frequency-modulated signal (hereinafter referred to as an FM signal) using a semiconductor laser and optical heterodyne detection.
2. Description of the Background Art
FIG. 7 is a block diagram showing the structure of a conventional FM modulator. The FM modulator with this structure is shown, for example, in a reference (K. Kikushima, et al, “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, IOOC '95 Technical Digest, Vol. 5, PD2-7, pp. 33-34). In FIG. 7, the FM modulator includes a signal source 600, a driving amplifier 602, a frequency modulation laser (hereinafter referred to as an FM laser) 604, a local light source 605, and an optical-electrical converting portion 606.
In the above structured FM modulator, the signal source 600 outputs an electrical signal which is an original signal for FM modulation and the driving amplifier 602 amplifies the electrical signal. The FM laser 604, which is structured of a semiconductor laser element and the like, for example, oscillates light having a wavelength λ1 on condition that an injection current is constant. When the injection current is amplitude-modulated, the outputted light is modulated in an oscillation wavelength (optical frequency) as well as in intensity, and the FM laser 604 outputs an optical frequency-modulated signal having the wavelength λ1 at the center. The local light source 605 outputs unmodulated light having a wavelength λ0 which is different from the oscillation wavelength λ1 of the FM laser 604 by a prescribed amount Δλ1. The outputted optical signal from the FM laser 604 and the outputted light from the local light source 605 are combined to be inputted to the optical-electrical converting portion 606. The optical-electrical converting portion 606 is structured of a photodiode having a square-law detection characteristic, and the like, and generally has the properties of converting an optical intensity modulation component of the inputted light into a current amplitude modulation component (hereinafter referred to as an optical intensity modulation/direct detection component: an IM-DD component) and, when two lightwaves having different wavelengths are inputted, generating a beat component of the two lightwaves at a frequency corresponding to the wavelength difference (this operation is called an optical heterodyne detection). Accordingly, the optical-electrical converting portion 606 outputs the beat signal of the outputted optical signal from the FM laser 604 and the outputted light from the local light source 605 at a frequency corresponding to the wavelength difference Δλ between the two lightwaves.
The beat signal obtained as described above is an FM signal taking the electrical signal from the signal source 600 as an original signal. Therefore, by using the appropriate FM laser 604 and local light source 605, it is possible to easily generate a high-frequency and wide-band FM signal having a center frequency (carrier frequency) more than several GHz and frequency deviation more than several hundred MHz, which it is difficult to realize in an FM modulator with an ordinary electric circuit.
In the conventional FM modulator having the above structure, a carrier-to-noise ratio (hereinafter referred to as a CNR), which shows the quality of the FM signal, is improved as the frequency deviation in the FM laser 604 increases and as spectral line widths of the FM laser 604 and the local light source 605 become narrower. The spectral line widths of these two light sources are parameters depending on the composition and structure of each light source and cannot be changed greatly by limitations such as use conditions and the like. However, if the amplitude of the inputted signal to the FM laser 604 is increased, it is possible to increase the frequency deviation and thus improve the CNR. However, since the laser light source has a threshold characteristic, when the amplitude of the inputted signal is increased to more than a prescribed degree, a distortion characteristic is extremely deteriorated due to clipping of the signal amplitude and the like. Furthermore, the outputted signal level of the driving amplifier 602 has a limit (saturated level), and if the outputted signal level is increased over a prescribed level, the distortion characteristic is sharply deteriorated. Therefore, the amplitude increase of the inputted signal to the FM laser 604 is limited, and it is thus disadvantageously difficult to improve the CNR to more than a prescribed degree.